The invention relates to a thin film contact dryer having a rotor. The moist material introduced is repeatedly spun on to the heated internal wall of the dryer, by the rotor, until it can finally be removed at the other end of the dryer in a dry, flowable form.
When moist materials are fed into a thin film contact dryer as a pumpable paste, relatively large agglomerates of product (lumps) form which are interspersed in the otherwise powdered dry material. These lumps are about the size of peas or hazel nuts. With a given granulation tendency, lump formation is also possible with flowable moist materials.
The lumps, which are still moist inside, make the dry material unusable as it does not meet the requirements concerning residual moisture, grindability and handling. Sieving and recirculating the lumpy fraction is very expensive. It has therefore been impossible hitherto to use the horizontal thin film contact dryer in cases where even small quantities of lumps are formed.
Accordingly, revolving distributing elements which are bent back have been developed which are intended to prevent the lumps from forming since the product is drawn into a conical gap between the distributing elements and the cylindrical heating surface (at rest) and are spread. It has been found, however, that although a significant reduction could be achieved in the lumpy fraction of the dry material, the reduction was never sufficient.
Another suggestion involves slanting the dryer to increase the product residence time to such an extent that the lumps are destroyed. However, this results in unpermissible mechanical stresses due to the increased quantity of product in the dryer.